1. Field of the Invention
This invention relates to measuring air flow.
2. Prior Art
Known airflow meters include a hot film anemometer sensor. In accordance with such hot film wire or hot film anemometry, a thin film wire sensor is deposited on the substrate such as quartz or glass. It is also known to use a very fine platinum or tungsten wire freely supported or wound on a ceramic bobbin and maintained at a certain temperature above the intake air temperature by electronic sensing and feedback circuits. Any change in the airflow alters the cooling effect of the air on the heated wire. An electronic circuit can sense this change in heat transfer rate and record changes in the heating current to maintain the temperature of the wire at a set value.
Hot film anemometer sensors have generally had a low speed of response as a result of the low thermal conductivity of the quartz glass or fiber. Further, great care must be taken in handling the fine film coated quartz fiber in manufacturing the sensor elements . Connecting the quartz glass fiber or fine wire to a supporting structure and making electrical contact involves time consuming and delicate operation. Electronics used to amplify and power the sensor are placed external to the hot film anemometer sensor and interconnection between the electronics and the sensor is made by wires. The connections between the sensor element and the interconnection wires are a source of loss of reliability and increased unit costs. This results in limited manufacturing production capacity and increased unit costs.
U.S. Pat. No. 4,129,848 issued to Frank et al teaches a platinum film resistor device. A layer of quartz, deposited upon an insulative substrate, is sputter etched to produce etched pits in the surface thereof. A layer of platinum is deposited over the quartz layer. A second layer of quartz is deposited over the layer of platinum and the second layer of quartz is masked and chemically etched away in the regions where the platinum layer is to be removed. The exposed platinum and a portion of the second quartz layer are then sputter etched away leaving the platinum in a predetermined configuration.
There still remains a need for an airflow sensor haviang a good thermal conductivity which can be made at a very low unit cost with high reliability. Further, it would be advantageous to have a sensor which reduces system costs and improve overall system reliability as well as increasing packaging efficiency by including signal and power electronics on the same substrate as the airflow sensor element. These are some of the problems this invention overcomes.